DE-A 29 43 833 discloses the use of thiourea and of thiourea derivatives as additives in an aqueous conversion coating composition which comprises free sulfuric acid, hydrogen peroxide, and silica. Treatment is principally with phosphorus compounds; thioacetamide is included in the description as a co-constituent.
All that is explicitly disclosed is the immersion of substrates to be coated in a bath at between 20 and 35° C. with subsequent hot-water rinsing in the range between 60 and 70° C. and subsequent drying at temperatures above 70° C.
EP-A 878 519 discloses preferably chromium-free, aqueous compositions for producing corrosion control coatings, comprising 0.2 to 50 g/l of a thiocarbonyl compound, 0.1 to 5 g/l of phosphates, and water-soluble binders or binder dispersions. 10 to 500 g/l of SiO2 may optionally be present. The thiocarbonyl compounds may be, for example, thiourea, thioamides, thioaldehydes or thiocarboxylic acids.
The corrosion coatings described therein require heating and drying of the coated material in a hot environment, or coating of a substrate which has been heated beforehand. This requires temperatures between 50 and 250° C. Only coat thicknesses of 0.1 to 20 μm are disclosed; coat thicknesses outside of this range are described as being ineffective or uneconomic.
JP-A 2002-64856, JP-A 2002-241957, JP-A 297384, and JP-A 2003-73856 disclose various kinds of aqueous compositions for producing corrosion control coatings, comprising aqueous different aqueous polymer dispersions, further components, and also thiocarbonyl compounds, such as thiourea or ethanethioamide, for example. Thioamides containing more than one thioamide group are not disclosed for atmospheric corrosion control.
From the prior art recited it is apparent that thioamides have to date been disclosed predominantly in coil coatings, by which are meant coatings on rolled metal strips which following their production are wound up into rolls (called coils) for storage and transportation. These metal strips constitute the starting material for the majority of sheetlike metallic workpieces, examples being automobile parts, bodywork parts, instrument casings, exterior architectural facings, ceiling panels or window profiles. For these purposes the appropriate metal sheets are shaped by means of appropriate techniques such as punching, drilling, folding, profiling and/or deep drawing. Relatively large components, such as automobile bodies, for example, are assembled if appropriate by the welding together of a number of individual parts.
Coil coating is the continuous coating of metal strips with usually liquid coating materials. Metal strips with a thickness of 0.2 to 2 mm and a width of up to 2 m are transported at a speed of up to 200 m/min through a coil coating line, and are coated in the process. For this purpose it is possible, for example, to use cold-rolled strips of soft steels or construction-grade steels, electrolytically galvanized thin sheet, hot-dip-galvanized steel strip, or strips of aluminum and/or aluminum alloys. Typical lines comprise a feed station, a strip store, a cleaning and pretreatment zone, a first coating station along with baking oven and downstream cooling zone, a second coating station with oven, laminating station, and cooling, and a strip store and winder.
Characteristic of coil coatings are thin coats of the coating materials which have a dry coat thickness of usually well below 80 μm, often below 60 μm, below 50 μm, and even below 40 μm. Moreover, the metal sheets are processed with a high throughput, necessitating short residence times; in other words, necessitating drying at elevated temperature following application of the coating, in order to make the coating material durable quickly.
Coating processes of this kind can of course be carried out only on lines produced specifically for that purpose, with the consequence that existing constructions, for example, cannot be coated with processes of this kind.